This Seaweed-Covered House Is the World’s Coziest Sushi Roll

The ceiling panels are filled with seaweed, wrapped in linen, and mounted on MDF boards prior to installation. Photo: Helene Høyer Mikkelsen/Realdania Byg

One material, two very different styles, both uniquely Danish. Photo: Helene Høyer Mikkelsen/Realdania Byg

The primary challenge for the designers was turning an unruly weed into a consistent building material, a feat achieved by stuffing the material into a tubular net. Underneath the seaweed pillows the facade walls are prefab timberframe panels clad with OSB on both sides and covered with tar felt. Photo: Helene Høyer Mikkelsen/Realdania Byg

Realdania, the philanthropic group that funded the construction of the New Seaweed House is focused on preserving Danish architecture and providing funding for new construction based on cultural identity. Photo: Helene Høyer Mikkelsen/Realdania Byg

The interior is paneled with softwood boards wiped with white soap. The steep roof helps drain water effectively, but also creates extra space for visitors to sleep. Photo: Helene Høyer Mikkelsen/Realdania Byg

Seaweed is a surprisingly durable material, though left unchecked, small trees will lay roots in the thatched roof. After a year of exposure, the pillows take on the silvery colors of the local pine trees. Photo: Helene Høyer Mikkelsen/Realdania Byg

The house will be available for rent during the summer months—a perfect listing for Airbnb. Photo: Helene Høyer Mikkelsen/Realdania Byg

The seaweed pillows give the building its unique texture, but traditional building materials keep the structure water tight. The pillows and exterior walls are expected to last 20 years—longer than some asphalt shingles. Photo: Helene Høyer Mikkelsen/Realdania Byg

The history of modern architecture has been tightly coupled with technological development, from the Bessemer steel that provided structure for Louis Sullivan’s skyscrapers to CAD simulations that transformed Frank Gehry’s sketches into swirling, titanium clad landmarks. Yet in Denmark, this thoroughly modern house draws its unique feature not from a scientific advance, but from the era of Viking sagas.

The Modern Seaweed House is a 1100 square-foot, $360,000 vacation home on the Danish island of Læsø and incorporates seaweed as a building material in three ways. The most obvious is the array of cylindrical, seaweed-stuffed “pillows” on the building’s facade that take the place of traditional shingles or clapboards. The adaptable algae is also stuffed into the light pine walls and serves as a replacement for mineral wool insulation. The ceiling is padded with the curious construction material and provides insulation as well as acoustical dampening for the open floor plan.

The New Seaweed House is a collaboration between architecture firm Vandkunsten and Readania Byg an organization that spends millions of dollars each year preserving Denmark’s material cultural by restoring historical properties and funding the development of new experimental designs that reflect Danish sensibilities.

The seaweed provides a surface texture unlike any other building material. Photo: Helene Høyer Mikkelsen/Realdania Byg

“Seaweed is unruly and needs to be tamed,” says Søren Nielsen.

This project sprouted up after Realdania finished restoring a cottage with a traditional seaweed thatched roof. After getting direct experience with the material, the team wanted to see if the unruly, undersea feedstock could be used in a more modern design. “The idea is to revive interest for the unique tradition of seaweed thatching,” says Jørgen Søndermark, Project Manager and architect at Realdania. “And in a broader sense, re-introduce overlooked or disregarded organic materials at a time where low-carbon solutions are much called for.” The structure serves as an object lesson in how natural resources like seaweed can be integrated in a contemporary design and is intended to inspire others to explore alternative materials.

The main challenges was figuring out how to use the messy material in a way that would reflect Scandinavia’s modern style, not a medieval hut. “Seaweed is unruly and needs to be tamed,” says Søren Nielsen, the project architect from Vandkunsten. The solution is tubular wool net that seaweed is stuffed into transforming the wayward weed into a manageable building block. The seaweed pillows were created by hand for this project, but could be mechanized if there is sufficient demand from the architectural community.

The other major consideration was preventing moisture build up. The roof’s pitch is steep allowing rain water to drain quickly, the pillows are kept a few inches off the roof to provide airflow, and a layer of roofing felt provides the real waterproofing. The seaweed used as insulation inside the walls is kept try by exterior layers of wood and waterproofing materials. The concept sounds wild, but construction process follows all the local building codes.

This is the seaweed thatched cottage that inspired Realdania to consider the material as a modern building block. Photo: Helene Høyer Mikkelsen/Realdania Byg

Contrary to popular belief, seaweed doesn’t smell or itch like straw and the designers estimate the exterior pillows could last a hundred years, or more. According to the designers, they act like moisture batteries, pulling water out of the air during humid periods and releasing it as the air dries. The only time seaweed seems to fail is when its true nature is compromised. “The seaweed has to be handled as it is,” says Søndermark. “Attempts have been made previously to cut it short in order to use it for blown-in insulation but that failed because the seaweed lost its insulation powers and started to deteriorate.”

Seaweed doesn’t smell or itch like straw, and could last a hundred years or more.

The house is expected to last as long as any other building, but the architects hedge a bit, pointing out that their kelp construction process is unprecedented. Seaweed is robust and doesn’t sprout mold or attract pests, yet birds do tend to make nests in thatched roofs and given enough time, small trees may grow on the roof. Fortunately, a scheduled maintenance plan can keep this slick shelter from devolving into something from the Shire. Locals have no concerns about the building, seaweed has been used on the island for hundreds of years and building inspectors were happy to see that the material has been tested at labs in Germany receiving certificates of safety and efficacy.

The high level of insulation, airtight assembly, heat recovering system, and low-energy windows—not to mention the seaweed’s natural color—makes this a very green building. An independent lifecycle analysis estimates that the building has negative carbon footprint equal to 8,500 kilograms worth of CO2 and the designers expect the building will operate for 20 years before it has any appreciable energy impact.

The use of seaweed is a uniquely Danish innovation, but the team feels the design reflects their cultural sensibilities even looking past the curious wall covering. The lightly decorated open spaces, clean lines, and exposed wood are Scandinavian trademarks. The long, narrow floorplan is typical of the vernacular architecture on the island. However, Nielsen says there is another highly characteristic Danish design principle at play in the building: prefabrication. As he points out, “There is a reason that Lego is a Danish invention.”